Gold Oxide Nanoparticles with Variable Gold Oxidation State

Article pubs.acs.org/JPCC

Gold Oxide Nanoparticles with Variable Gold Oxidation State Maxim Tchaplyguine,*,† Mikko-Heikki Mikkela,̈ † Chaofan Zhang,‡,§ Tomas Andersson,‡ and Olle Björneholm‡ †

MAX-lab, Lund University, P.O. Box 118, 22100 Lund, Sweden Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

‡

ABSTRACT: Gold-oxide-containing nanoparticles have been produced in a range of partial to full oxidation conditions, where the nanoparticle electronic structure and stoichiometry have been characterized. Our results indicate that with the increase of the oxidation degree in these nanoparticles the gold oxidation state possibly changes from lower oxides with monoor divalent metal to the higher oxide with the trivalent gold. At intermediate oxidation conditions our observations are consistent with a radially segregated structure of such nanoparticleswith the core containing mainly oxide and the surface covered with few monolayers of metallic gold. These results have been possible to obtain combining the vapor aggregation method for the nanoparticle fabrication and synchrotron-based photoelectron spectroscopy for their characterization. The deposition of the oxidized nanoparticles has showed that the species assigned as containing lower oxide could be preserved in the landing and then studied on a substrate for a limited time. The possible lower oxide formation in nanoparticles is discussed in connection to the enhanced catalytic activity of gold nanoparticles.

1. INTRODUCTION Gold nanoparticles and their derivatives are currently at the center of close attention for several frontier-science fields; however, probably nowhere they are as “hot” as in catalysisrelated research.1−3 The chemisorption of carbon monoxide (CO) on gold has become one of the most studied surface processes ever.3−5 Over the years it has been vividly discussed what made gold nanoparticles so drastically different from macroscopic gold: the size, 5,6 the peculiar electronic structure,7,8 the interaction with a specific substrate,9,10 etc., and a clear answer seems to have not been found so far.5,6 The catalytic process discussed most is the oxidation of carbon monoxide at the gold nanoparticle surface in the presence of oxygen. This reaction proceeds via a formation of the surface oxide of the catalystof the gold. Among the main obstacles to our understanding of the gold reactivity enhancement in nanoparticles are the difficulty to oxidize this noble metal at laboratory conditions and the instability of its oxide. At the macroscale one of the most efficient methods to create gold oxide is metal surface treatment by oxygen-containing plasma.11−13 After the exposure to such plasma the gold surface shows to be textured within several nanometers into the bulk, where the presence of the most typical gold oxide Au2O3is detected as a rule.11−13 Electrochemical oxidation has been seen to lead to either trivalent (Au3+) or lower (Au2+, Au2+) oxides of gold.14,15 The oxidation state of gold has been possible to establish with the help of X-ray photoelectron spectroscopy (XPS) which detects well-separated responses of the oxide and metallic gold in the Au 4f binding energy region. © 2015 American Chemical Society

The trivalent oxide response in a photoelectron spectrum is separated by 1.8−2.1 eV from the metal signal.11,13,16,17 In some works a smaller spectroscopic separation of 1.4−1.5 eV from the parent metallic response has been observed and interpreted as due to the lower oxidesAuO and/or Au2O (see, e.g., ref 13 and references therein). The gold valency in an oxide may come into play when the adsorption of CO molecules on goldin the presence of oxygenis about to take place. As mentioned above, the surface gold oxide formation is a precursor of the CO conversion into CO2 (see, e.g., ref 2). A CO molecule is a two-electron donor in a reaction with noble metals,4,18 so if the substance to which CO has to bind contains divalent metal the situation is optimal for the CO chemisorption. Experiments4,19 performed on free noble-metal clusters consisting of a few to few tens of atoms have showed stronger interaction of the clusters with CO when the cluster electronic configuration could accommodate just two electrons from a CO molecule. In other words, the clusters with “divalent” metal have been shown to be chemically more active, illustrating that the peculiarities of the gold oxidation state in clusters/nanoparticles are of relevance for catalysis. The present work has been aimed at fabrication and characterization of gold-oxide-containing nanoparticles in the size regime known to be efficient for catalytic oxidationbelow Received: January 26, 2015 Revised: March 27, 2015 Published: March 31, 2015 8937

DOI: 10.1021/acs.jpcc.5b00811 J. Phys. Chem. C 2015, 119, 8937−8943

Article

The Journal of Physical Chemistry C

Figure 1. Schematic representation of the experimental setup. The nanoparticle beam comes out from the cylindrical cryostat through a 30 mm long copper nozzle with a 2 mm channel diameter. The cryostat is attached to an XYZ manipulator. The horizontally polarized X-ray radiation crosses the nanoparticle beam at 90°. The photoelectrons, emitted with a certain angular distributionspecific for each energy level in questionare detected by an electron spectrometer (not shown) with a narrow acceptance angle (≈10°) along and perpendicular the light direction. In the present experiment the acceptance axis of the spectrometer has been at 90° to the polarization plane of the radiation.

geometry and typical Ar−He pressures used in our experiment form nanoparticles of a few nanometers in diameter.27 The type of the metal influences the dimensions only within the defined above interval. In a preliminary series of experiments the optimal conditions for the fabrication of nonoxidized gold nanoparticles have been established. The corresponding electronic structure of free, unsupported gold nanoparticles coming out in a beam from the cryostat has been characterized by XPS. For this characterization the nanoparticle source has been attached to the experimental station of the I411 beamline of the Swedish national synchrotron radiation facility Max-lab in Lund. The beamline provides the X-ray radiation in a wide range starting from hν = 40 eV and lasting up to several hundred electronvolts with sufficient radiation flux. Such a range conveniently includes the energy at which the gold 5d valence level has the highest ionization cross-section (hν ≈ 40 eV), as well as hν ≈ 200 eV where the cross-section of the Au 4f level (with electron binding energy of ≈90 eV) finally recovers from its deep minimum at hν ≈ 100 eV.28 The beam of nanoparticles is crossed by the ionizing radiation at 90° (Figure 1), and the ejected photoelectrons are detected and energy-analyzed by an electrostatic hemispherical electron spectrometer (Scienta R4000). The nanoparticles coming out from the cryostat can be probed by XPS “on the fly”in the beam, without deposition on a substrate. Such an option allows avoiding the effect of the substrate on the particles, so the information on the inherent properties not obscured by the macroscopic support can be obtained. In the present experiments the electron spectrometer has been fixed with its acceptance axis at 90° to the horizontal polarization plane of the beamline radiation. This geometry influences the relative representation of the inner (bulk) part of the nanoparticles and its surface monolayer in the XPS signal. As a result, certain judgments on the chemical composition of the surface monolayer and of the bulk can be made, which is discussed in the Results section. In a separate series of experiments, oxide-containing nanoparticles have been deposited from a beam on a naturally oxidized p-doped (boron) Si(111) substrate (resistivity 10−20 Ohm·cm) at room temperature. The deposition time has usually been less than 5 s and has been controlled by a

10 nm (e.g., see ref 5 and references therein). The nanoparticle fabrication method used in our work is based on vapor aggregation, involving metal atoms sputtered off the solid gold in the oxygen-containing plasma. For the oxide-containing nanoparticles the question of oxidation state/valency has been addressed using X-ray photoelectron spectroscopy.

2. EXPERIMENTAL SECTION The vapor aggregation method involving magnetron reactive sputtering in argon−oxygen atmosphere has recently allowed us to tune the degree of oxidation and the distribution of components (metal versus its oxide) in nanoparticles out of several metals.20−22 In these works, at certain fabrication conditions, core−shell structured nanoparticles have been createdwith the oxidized core covered by a few atomic layers of the parent metal. This specific structure has been realized due to the peculiarities of the fabrication method: the oxidation takes place at an early stage of aggregation close to the sputtered metal surface where dissociated, ionized, and excited oxygen is most abundant. Reactive magnetron sputtering with subsequent deposition of the oxide molecules is a well-established technique for producing metal oxide thin films (e.g., see ref 23) and was recently used also for gold oxide films.24 In our apparatus the magnetron is placed inside a liquid-nitrogen-cooled cryostat, and the aggregation path inside it is about 20 cm long. Further away from the magnetron the concentration of reactive species decreases drastically, so the formation of metal−oxygen complexes becomes much less probable. It is here where the oxide core is gradually covered by a metallic layer.20,21 In our earlier works20,21 practically all metal atoms in nanoparticles were oxidized at higher oxygen fractions in the sputtering gas mixture. In the present work solid gold has been vaporized by sputtering in an argon−helium atmosphere. A 3 mm thick gold disc (the so-called target) has been mounted on a 1.3″-diameter magnetron. The cryostat has been attached to an XYZ manipulator and placed inside a vacuum chamber. In our apparatus (called further the nanoparticle source) a continuous cold gas flow through the cryostat shapes a beam of nanoparticles with the dimensions depending mainly on the aggregation time spent inside the cryostat.25,26 The fixed 8938


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macroscopic polycrystalline gold separation. The spin−orbit splitting value is an informative magnitude related to the atomic coordination, as will be discussed below. The Au 6s band sets at ≈5 eV below the vacuum level, thus defining the nanoparticle work function. This onset of the valence band is very close to at least some values of the macroscopic gold work function found in the literature.31 The difference of the order of 0.1 eV with the macroscopic gold is an indication that the nanoparticles are metallic and several nanometers in diameter: it is this difference in the electron binding energies between the nanoparticles and macroscopic solid which contains the information on the nanoparticle dimensions.25,27,32 To know the work function of nonoxidized species (or the valence band onset), is also important when the valence spectra of the gold-oxidecontaining nanoparticles are discussed. In the measurements on the particle beam there is an advantage of heaving an absolute binding energy scale. All the energies are referenced then to the so-called vacuum level. Due to the availability of such a scale the change in the onset of the valence band in the oxide can be directly correlated to the absolute binding energy of the top of the valence band for this binary compound. This energy varies with the change of this compound composition, as we will see below. As for the Au 4f core level, the peculiarity of this level of ionization is in the narrow, atomic-like responses of the metal, with the binding energy positions and spectral shapes carrying information on the local chemical environment. This is what often leads to the separation of the photoelectron responses from the atoms of the same element but surrounded either by identical atoms or having, for example, oxygen atoms in some coordination. Such separation is possible also for nanoparticles of just a few nanometers in diameter.20,21 Also the responses of the same-element atoms in the bulk and at the surface can appear at different positions in the spectra, even at such nanoscale dimensions.27 In the present experiments the Au 4f spectrum of nonoxidized gold nanoparticles has been shown to consist of two lines separated by a typical value for the gold metal, ≈1.7 eV. These two lines are known to be due to the 4f7/2 and 4f5/2 spin−orbit components. For our nanoparticles the spin-orbit character of the splitting manifests itself in the close to statistical 4:3 intensity ratio for the two components (Figure 2a, right panel). The lower-energy 4f7/2 spin−orbit component in the nanoparticle spectrum has been detected at ≈89 eV, which corresponds to the expectations: relative to the Fermi edge the macroscopic solid Au 4f7/2 binding energy is known to be 84 eV. The gold work function is ≈5 eV, which gives ≈89 eV for Au 4f7/2 relative to the vacuum level. Detailed analysis and discussion of the nanoparticle response in the Au 4f region are necessary to identify and adequately interpret the complex cases presented further down, the cases when both metallic and oxidized gold coexist in the particles and their fractions change with the increase of oxygen concentration in the sputtering gas mixture. Our previous experiments on nanoparticle beams produced using the same vapor aggregation source bear a witness of not only the binding energies but also the spectral shapes of the core-level responses being very close to those of the corresponding macroscopic metals.20−22 Though a certain size distribution is present in the nanoparticle beam created by our apparatus, in the few nanometer size range the spread of the electron binding energies for different sizes in the distribution is as a rule well below the inherent spectral

rotational leaf shutter. In the XPS measurements on the supported sample the spectrometer has also been positioned at 90° to the horizontal polarization plane. The substrate has been mounted with its normal in the vertical plane, and the angle between the substrate plane and the radiation propagation direction has been 15°.

3. RESULTS AND DISSCUSION 3.1. Nonoxidized Nanoparticles. First, the photoelectron spectra of the nonoxidized gold nanoparticles propagating in a beam have been recorded in the valence and in the Au 4f corelevel regions. Figure 2a (left panel) presents a typical valence

Figure 2. Left panel: valence photoelectron spectra of gold nanoparticles recorded at hν = 40 eV. (a) Metallic particles. The 5s band lasts from 5 to 7 eV, and at 7 eV the 5d band sets on and lasts to 14 eV. The 5d band maxima are separated by ≈4 eV. (b) Gold-oxidecontaining nanoparticles fabricated with ≈1.5% O2 in Ar−O2 mixture; (c) with ≈2% O2 in Ar−O2; (d) with ≈4% O2. In spectra c and d gaseous oxygen response is seen between 12 and 13 eV. The 5d spin− orbit splitting decreases with oxidation degree. Right panel: Au 4f photoelectron spectra of gold nanoparticles recorded at hν = 200 eV. (a) Metallic particles: the fitting reflects the bulk and surface responses. (b) Gold-oxide-containing nanoparticles fabricated with ≈1.5% O2 in Ar−O2 mixture; (c) with ≈2% O2 in Ar−O2; (d) with ≈4% O2. The extra doublet from the oxidized gold is shifted up in (c) by ≈1 eV. In spectrum (d) the oxide is ≈1.5 eV above the metallic gold response.

spectrum of such nanoparticles, and their response in the Au 4f binding energy region is shown in Figure 2a (right panel). In both regions the spectra resemble to a great extent those of macroscopic polycrystalline gold (see refs 29 and 30 for example). Two main maxima at about 7.5 and 11.5 eV are as a rule assigned to the spin−orbit splitting of the 5d band. In the present case these maxima are at the characteristic for 8939


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intense beam of metallic Au nanoparticles has been achieved. The “on-the-fly” channel of the nanoparticle signal monitoring has been the valence Au 5d band. It occurred to have sufficiently high ionization cross-section to allow observing the spectral changes caused by each new portion of oxygen in “real time”. For that it has been enough to accumulate the signal for one secondusing a special acquisition mode of the electron spectrometer. At each new O2 fraction in the sputtering mixture a valence and a Au 4f spectrum have been recorded using the normal acquisition mode (Figure 2 left and right panels). Already with just a bit more than 1% oxygen (of the input pressure) in the Ar−O2 mixture the onset and the shape of the valence spectra were significantly different from those of metallic gold nanoparticles. The following changes were observed: the onset of the valence band shifted up in binding energy by ≈0.6 eV, and some additional intensity arose above the originally flat 6s band. This extra intensity is usually assigned to the formation of hybridized Au 6s−O 2p orbitals of the oxide. One more change was in the shift of both maxima of the Au 5d band up by approximately the same value as for the valence band onset. At the next oxygen partial pressure pointwith just a bit higher O2 fractionthe oxide contribution to the spectrum increased to a degree where no characteristic for metallic gold stepwise transition from the Au 6s to Au 5d band was observed in the spectrum (Figure 2c, left panel). By its smooth rise the corresponding spectrum resembles the spectral shape recorded for macroscopic gold oxide samples.11 Also the spin−orbit splitting in the Au 5d band decreased relative to the metallic case. This decrease in the noble-metal compounds is usually assigned to the reduction in the coordination between the metal atoms of the same type.37 Thus, this decrease is one more indication of the gold oxide formation. Moreover, it also means that it is not two types of particlesthe metallic ones and those containing oxidized goldwhich coexist in the beam but that the beam is dominated by one type of particles with a specific new coordination of gold to oxygen. The spin−orbit splitting in the nanoparticles (case 2c) does not become as small as in the published spectra for the Au2O3 oxides where it is ≈2 eV.11 For a similar case of silver-oxide-containing nanoparticles produced by the present method we have recently shown that the splitting in the Ag 4d valence band gradually decreased by a factor of ≈3 with the degree of oxidation.22 In the case of gold nanoparticles the larger than Au2O3 splitting in the Au 5d band should indicate a higher Au−Au coordination and thus a lower oxide. When the oxygen fraction in the sputtering mixture reaches ≈4% the valence spectrum loses all its structureit adopts a smooth monotonous shape without any maxima (Figure 2d, left panel). The presence of gaseous oxygen in the nanoparticle beam becomes more obviousthe characteristic vibrational progression between 12 and 13 eV gains in intensity. An observation worth noticing is the further upward shift of the valence band onsetby almost 0.5 eV. However, without a corresponding Au 4f spectrum it is difficult to say something definite about the oxidation degree here. 3.3. Au 4f Photoelectron Spectroscopy on GoldOxide-Containing Nanoparticles, Lower Oxidation Degree. Figure 2 (right panel) presents a series of the spectra recorded in the Au 4f region using hν = 200 eV at the same fabrication conditions as the corresponding valence spectra in the left panel. While already at the lowest O2 fraction the oxide response is undoubtedly detected in the valence region (case

width of the core-level responses for any particular size. This allows us to analyze the core-level photoelectron spectra of the nanoparticles in the beam assuming the typical spectral characteristics established for the macroscopic gold: the spin−orbit splitting; the lifetime-defined inherent width; the asymmetry due to the metallic band structure causing the socalled Doniach−Sunjic spectral profiles;33 as well as the bulk− surface separation and mutual position (bulk response is at higher binding energy in gold30,34). Au 4f ionization is the case for which the bulk and surface responses are, in fact, not resolvedsince for the 4f peaks the lifetime defined width is ≈0.32 eV and the separation between the bulk and surface responses is ≈0.40 eV.30 In the spectral analysis one nevertheless assumes two (bulk and surface) correspondingly separated peaks (Figure 2a, right panel). The lifetime width, the bulk−surface separation, and the instrumental broadening together explain well the observed Au 4f total spectral width of ≈1.0 eV. The control of the spectral width is important recollecting that in some cases the noble-metal oxides manifested themselves as additional broadening of the main “metallic” peaks. If the instrumental contribution of the present experiment (≤0.2 eV) is subtracted from the total width, the remaining width indeed occurs to be again close to the solid macroscopic value. This result additionally justifies our numerical spectral analysis and the conclusions following from it. The ratio between the bulk and surface intensities cannot be unequivocally established in our experiments; however, there are considerations which allow us to expect the surface response to dominate in the Au 4f spectra of nanoparticles here. The kinetic energy of the photoelectrons ejected from the Au 4f level ionized with hν = 200 eV photons is ≈100 eV, which corresponds to the smallest electron escape depth in the whole range of kinetic energiesof about 4 Å.35 (This situation is special for goldfor many other elemental solids the minimum escape depth is reached at 2−3 times lower kinetic energies.35) Such escape depth of 4 Å is practically equal to the interatomic distance in solid gold. Thus, the probing depth of the method in such a case (Au 4f ionization, photoelectron kinetic energy ≈100 eV) is just one to two atomic monolayers under the surface. In its turn this means that the surface-monolayer signal should dominate the 4f spectrum recorded with 200 eV photon energy. The anisotropy of the 4f electron angular distribution at hν = 200 eV is low, so it should not significantly influence the relative bulk-surface responses of the nanoparticles.36 Both considerationsof the electron escape depth and of the angular anisotropywill be later also taken into account in the analysis of the gold oxide spectra. At this point one can also mention that there is an important difference in the probing depthbetween the Au 4f spectra recorded with hν = 200 eV and the valence spectra recorded with hν = 40 eV. The photoelectron escape depth for Au 5d at hν = 40 eV is about two times higher than for the Au 4f at hν = 200 eV,35 thus at least several more “bulk” monolayers contribute to the valence signal in comparison to the Au 4f case. In other words our valence spectra reflect much more the composition of the bulk than our Au 4f spectra. As will be discussed below, these considerations allow us to consistently explain the valence and the 4f spectra containing gold oxide response. 3.2. Valence Photoelectron Spectroscopy on GoldOxide-Containing Nanoparticles, Lower Oxidation Degree. In the present series of experiments oxygen has been added stepwise starting at the fabrication conditions when an 8940


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deposition of the nanoparticles with a comparable representation of both metal and its oxide with now clearly observed 1.5 eV separation between them in the gas-phase spectra, like in Figure 2d. Figure 3 presents the result of such a landing

2b), in the corresponding Au 4f spectrum the only clear change is the upward shift in binding energy of a “metallic” doublet. The value of the 4f level shift is about the same (0.6 eV) as for the valence band onset at the same conditions. Judging from these shifts’ similarity the 4f response in Figure 2b is due to the metallic gold in the nanoparticles. The change in the metallic response binding energy is probably due to the influence of the neighboring oxide areas, which do not yet manifest themselves in the 4f spectrum. As mentioned above, in the Au 4f spectrum we see mainly the surface monolayer response and the attenuated signal from one or two quasi-spherical atomic layers under it. Thus, judging from the Au 4f spectrum, the few outermost monolayers of the nanoparticles remain metallic in the case of 2b. This should also be the composition at the next step in oxygen concentration (case 2c)we still see a strong “metallic” doublet at about the same position as in case 2b. However, the oxide responseappearing as another doublet shifted by ≈1 eV to the higher binding energyis also seen now. (A more accurate estimate of the shift would not be trustfulin the view of the absence of the clear resolution between the two doublets.) This signal is likely due to the oxide which is just a few monolayers under the surface of the nanoparticlesagain in the view of the probing-depth considerations: The small escape depth explains a weaker oxide response in the Au 4f spectrum than in the corresponding valence spectrum. As discussed in the Introduction, the shift magnitude of 1.5 eV and smaller has been previously associated with the lower than Au2O3 oxideswith either AuO or Au2Oand it has not been possible to assign this spectral signature more exactly. There has been, however, a relevant study38 allowing us to make a certain hypothesis on the chemical formula of the oxide formed in our case. In this study the gas-phase gold oxide molecules were created by gold reactive sputtering in an Ar−O2 mixture, and the dominance of the AuO molecules has been established by mass spectroscopy. Thus, at least at certain conditions reactive sputtering does typically create divalent gold oxide. It seems then not unreasonable to assume that such a composition can be realized in our case of reactive sputtering. Even the most common at macroscale, Au2O3 oxide, is known to be unstableit disintegrates at rather modest heating or irradiation.11 The lower oxides have been even more difficult to preserve intact for a time long enough for adequate studies. In the method described in the present work a lower gold oxide in nanoparticles is likely to be protected with a gold−metal monolayer or two, thus a principle possibility appears to study it in deposited nanoparticles. (In a beam the instability problem is overcome by a continuous renewal of the sample.) For the supported particles it is advantageous to have the coexistence of metallic and oxidized parts in one and the same particlein the sense that a local, on-the-particle, energy calibration is then provided. If one attempted to deposit fully oxidized nanoparticles, the detection of only one Au 4f doublet in the supported-case spectrum would not obligatorily mean that the oxide was intact after the landing. Indeed, the nanoparticles can lose most of their weakly bound oxygen due to the heating induced by their collision with the surface.39 Then there would be only one doublet detectedhowever, not due to the oxide but due to the metaland the absolute energy calibration possible for separate supported nanoparticles is too uncertain for a univocal conclusion concerning the nature of only one Au 4f doublet in such a case. Taking into account all these considerations it seemed to us advantageous to attempt the

Figure 3. Example of Au 4f photoelectron spectra of deposited (on naturally oxidized Si(111)) nanoparticles containing gold oxide, fabricated with ≈4% in Ar−O2 mixture. Spectrum recorded at hν = 200 eV (not calibrated).

attempt: the spectrum of deposited particles recorded at the same hν = 200 eV within a few minutes after the deposition. The spectrum contains two doublets similar to those of the nanoparticles in the beam just before the deposition, so the lower-energy doublet can be assigned to metallic gold. The oxide doublet has a weaker relative intensity than before the deposition; however, the separation between the doublets is about the same 1.5 eV as in the beam for free nanoparticles. The same separation speaks for the fact that the chemical composition has been to a large extent preserved in the landing and that the coordination or the oxidation state remained mainly unchanged. However, within half an hour after the first spectrum was recorded there was only one lower energy doublet left. Several spectra measured during this period of time show how the oxide response gradually disappears. Nevertheless, the results for the deposited nanoparticles can be considered as valuable since the possibility of nondestructive landing of the nanoparticles with the weakly bound gold oxide has been demonstrated using XPS. 3.4. Photoelectron Spectroscopy on Gold-OxideContaining Nanoparticles, Higher Oxidation Degree. In our work on silver-oxide-containing nanoparticles22 we have shown that the increase of oxygen fraction in the sputtering mixture could lead to a change in the metal oxidation state. This observation motivated further experiments on gold oxidein an attempt to create gold nanoparticles with trivalent metaland, indeed, the continuation of our experiments has showed that the change in the oxidation degree seemed to have happened already at a rather small increase of the oxygen fraction in the sputtering gas mixture. This transition has manifested itself in the appearance of a larger separation, −1.8 eV instead of 1.5 eV, between the metal and the oxide responses. Such a situation is illustrated in Figure 4 where the valence and the core-level spectra for two different oxygen-gas concentrations are shown: (a) for the O2 fraction when the larger shift was first observed and (b) for the O2 concentration when finally no metallic response was anymore detected. In case (b) of complete oxidation the valence band onset takes 8941


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in nanoparticles at certain conditions can be relevant for the enhanced catalytic activity. As discussed in ref 5 this enhancement can be due to a combination of factors, with a certain hierarchy of them. One of such factors, which can facilitate the lower oxide formation, can be oxygen concentration, and this type of oxide might be advantageous for, for example, CO oxidation. The fabrication and precharacterization methods suggested in the present work can be seen as a novel way to study the nanostructured systems relevant for catalysis. Some of such systems, like studied here gold-oxide-containing nanoparticles, may exist only dynamically at industrial conditions, so the “onthe-fly” XPS allowing us to probe these weakly bound systems at laboratory conditions shows to be a unique characterization method.

Figure 4. Left panel: valence at hν = 40 eV. Right panel: Au 4f photoelectron spectra (hν = 200 eV) of nanoparticles containing gold oxide, fabricated with ≈6% in Ar−O2 mixture (upper spectra) and ≈30% O2 in the mixture (lower spectra). Referenced to vacuum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46462221491.

place at more than 7 eV, which is probably caused by the fact that the surface layer is also oxidized now. As discussed in the Introduction the 1.8 eV or larger separation between metallic and oxidized gold responses has always been taken as the indication of gold trivalency (Au3+) in the oxide, also in nanoparticles produced by various methods.40,41 A smaller separation of 1.5 eV, observed in few cases for oxidized macroscopic gold samples, has been as a rule assigned to lower gold oxides.13 Relying on these earlier assignments we suggest that in the present work a possibility to change gold oxidation state in nanoparticles, by controlling the doze of oxygen, has been demonstrated.

Present Address §

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work presented here has been supported by the Swedish Research Council (VR), the Göran Gustafsson Foundation, the Knut and Alice Wallenberg Foundation, the Crafoord Foundation, Nordforsk, and the Swedish Foundation for Strategic Research. Ch. Zhang would like to acknowledge the China Scholarship Council (CSC) and National University of Defense Technology (NUDT) for the graduate fellowship. We would also like to thank the MAX-lab staff for their assistance.

4. CONCLUSIONS In the present work a nanoparticle fabrication method has been developed which allows controlling the gold oxidation degree and the radial distribution of metallic and oxidized gold in the particles of a few nanometer dimensions. The changes in the chemical composition have been possible to disclose and monitor using core−level and valence photoelectron spectroscopy with synchrotron radiation. The composition control and tuning could be performed using “on-the-fly” photoelectron spectroscopy which has been probing the nanoparticle beam before any deposition on a substrate. The comparative analysis of the valence and Au 4f spectra speaks for the core−shell structure of the nanoparticles produced at intermediate oxidation conditions: with the core consisting of the oxide covered with few monolayers of metallic gold. For the explanation of the differences in the photoelectron spectra recorded at different oxidation conditions the following hypothesis has been suggested by us. Depending on the oxygen fraction in the gas mixture used for the nanoparticle fabrication the oxidation state can be changed from an oxide with mono- or divalent gold to that with trivalent gold. The latter is known to be the typical case for macroscopic gold oxide. The change in the chemical composition is also manifested in the valence spectra: The top of the valence band has been seen to shift toward higher binding energies from ≈5.5 eV to more than 7 eV. The lower oxide, known to be less stable than Au2O3, has been also possible to observe when the nanoparticles containing it were deposited from the beam on to a substrate. Among others, one of the questions evoked by our observations is whether the formation of the lower gold oxide

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REFERENCES

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Gold Oxide Nanoparticles with Variable Gold Oxidation State

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